Summary of Impact: Between 01:40 and 05:10 UTC customers may have experienced issues with queued messages in Exchange Online (EXO) due to Microsoft Issues.
Root Cause: Issues with Microsoft 365 meant that some messages were not being received by 365 after having been sent to Exclaimer. Microsoft have published the root cause to this issue under ID EX196121.
Mitigation: Microsoft determined that a change made to their 365 infrastructure was the cause of the problems and reverted the change.
Next Steps: Microsoft are reviewing their deployment procedures to help prevent similar problems in the future.
More information can be obtained directly from Microsoft by raising a case with
them against the ID quoted above.